1. Field of the Invention
The present invention relates generally to a method and composition for treating flue or exhaust gases to remove sulfur oxides and/or hydrochloric acids and, specifically, to a method in which a specially modified calcium hydroxide is brought into contact with the gases to be treated.
2. Description of the Prior Art
A wide variety of chemical processes, such as the burning of fossil fuels and metal ores, produce flue or exhaust gases (referred to herein as "off-gases") containing sulfur oxides and/or hydrochloric acids. Because of environmental concerns, much attention has recently been devoted to the control of emissions from such processes, particularly fossil fuel combustion. Attention has been directed especially to systems which permit retrofitting of existing installations to avoid the capital investment required to construct new facilities.
One attempted solution to the above problem is the use of wet absorption processes, commonly referred to as wet scrubbing. Such processes are known for treating acid gases by contacting the acid gases with an aqueous suspension of calcium hydroxide. Such processes are described, for example, in U.S. Pat. Nos. 3,914,378; 3,919,394; and 4,164,549, among others. These reactional processes produce a sludge containing calcium sulfite or sulfate. This sludge is a waste which is difficult to dewater, i.e., increase solids content, and to transport for further processing and/or disposal.
It is also known in the art to use "semi-dry" processes in which a calcium hydroxide suspension (milk of lime) is sprayed into the gases to be treated. When contacting the slurry with the hot gases to be treated, the suspension absorbs acid compounds and is dried. The by-product produced in this process is a fine, moist-to-dry powder. Despite this fact, the semi-dry process has, as a disadvantage, that it generally requires a substantial investment.
Because of the previously mentioned disadvantages, attention has been focused on the direct injection of dry absorbent into exhaust or flue gases containing sulfur oxides and hydrochloric acids. See, for example, U.S. Pat. No. 4,555,392, which teaches the injection of limestone and the injection of portland cement downstream in the combustion gas stream of a fossil fuel burner. Injection equipment is well known and conventional. Downstream injection of a "dry" absorbent would eliminate the requirement of substantial new capital expenditure or the requirement of substantial down-time of the furnace or associated equipment being retrofitted.
To be suitable for the above purposes, the selected dry absorbent must, in addition to being available as a dry powder, be characterized by a high reactivity, high utilization potential and a high capacity to absorb acid gases, especially SO.sub.2 and HCl, so that the powdered absorbent may be directly injected into the gases to be treated for obtaining a treatment which is both practical and efficient. Otherwise, the increase in the mass of solids to be separated from the off-gas stream, in most industrial processes, becomes impractical.
It has been observed that the capacity of calcium hydroxide powder to absorb acid gases is substantially proportional to its surface area (sometimes referred to as specific area). As a result, attempts have been made to find processes for preparing calcium hydroxide characterized by higher and higher surface areas.
In one known process, quicklime is slaked in the presence of methanol, thereby obtaining a calcium hydroxide having a surface area generally between 17 and 35 m.sup.2 /g. This manufacturing process is expensive and dangerous due to the production of methanol vapor. The bulk density, measured in kg/m.sup.3, of the obtained hydrate is low (typically about 0.3) so that its transport from the production plant to the installation for treating gases is very expensive.
In another process, quicklime is slaked in the presence of calcium lignosulfonate. The calcium hydroxide which can be obtained by this process has a low relative surface area and its efficiency is therefore limited.
In still another process, powders of quicklime or dolomitic quicklime are hydrated with water, the water to quicklime/dolomitic quicklime ratio being between 2:1 and 5:1, before being dried. The substantially dry powder obtained in this manner is then stored and transported to the installation in which the powder is injected in exhaust gases to be treated. The drying is an expensive operation which requires the investment of a dryer. Drying also agglomerates the powder particles and reduces their reactivity. The transport of the dry powder, due to its low bulk density, is also very expensive.
It is an object of the present invention to provide a composition and method for treating exhaust gases which avoids the drawbacks of the above referenced known processes and which is especially adapted for dry injection of a absorbent powder into an off-gas stream to be treated.
Another object of the invention is to provide a modified calcium hydroxide which is characterized by a high reactivity, high utilization potential and a high capacity to absorb acid gases, especially SO.sub.2 and HCl, so that the powdered absorbent may be directly injected into the gases to be treated for obtaining a treatment which is both practical and efficient.
Another object of the invention is to provide a method for preparing a calcium hydroxide suitably modified for the purposes of the present invention, at the site of utilization, without requiring expensive dryers, large storage vessels, and expensive transport.